1. Field of the Invention
The present invention relates in general to an ultrasonic flow or current meter and more particularly to an ultrasonic flow or current meter which is simple in circuit construction but can quickly measure with high accuracy.
2. Description of the Prior Art
In ultrasonic flow meters, there have been widely used a time difference method and a frequency difference method. The theory of these two methods, the construction of the ultrasonic flow meter used for practising the methods and their advantages and defects will be described briefly.
First, the theory of the time difference method will be described. When an ultrasonic wave propagates or transmits through a fluid, its apparent velocity of propagation differs depending upon whether the fluid is at rest or is moving. If it is assumed that the velocity of sound (ultrasonic wave) in a fluid at rest is taken as c and the velocity of a moving fluid is taken as u, the apparent velocity of the sound is (c + u) when the propagation direction of the sound coincides with the flow direction of fluid, but becomes (c - u) when the propagation direction of sound is opposite to the direction of flow of the fluid.
It is assumed that a pair of ultrasonic transducers (ultrasonic probes) P.sub.1 and P.sub.2 are mounted on a pipe P, through which a fluid F may flow, with a distance L therebetween and that an ultrasonic wave U is propagated along a direction with an angle .theta. to a flow direction FD of the fluid F in the pipe P as shown in FIG. 1. In this case, the propagation time interval of ultrasonic wave U emitted, for example, from the transducer P.sub.1 and arriving at the transducer P.sub.2 is t.sub.1 and the traveling time interval of the ultrasonic wave U emitted, for example, from the transducer P.sub.2 and arriving at the transducer P.sub.1 is t.sub.2, which are obtained from the following equations (1): ##EQU1##
Where, if the condition c.sup.2 &gt;&gt; u.sup.2 cos.sup.2 .theta. is satisfied, the following equation (2) can be derived from the equation (1): ##EQU2##
Accordingly, the flow velocity u of fluid F can be obtained from equation (2).
In fact, the time difference .DELTA.t between the time intervals t.sub.1 and t.sub.2 (.DELTA.t = t.sub.2 - t.sub.1) is very short, so that different methods have been considered for the measurement of such a short time difference. With this method, as apparent from the equation (2), the measured value of the flowing velocity u varies when the velocity c of sound in the fluid F changes. In general, the velocity of sound through a liquid or gas changes a great deal in response to the temperature changes of the liquid or gas, and further if foreign substances and/or bubbles exist in the liquid or the composition of the liquid changes, the velocity of sound therethrough changes much more. Therefore, it is necessary to compensate for such veocity changes.
With reference to FIG. 2, the time difference method of the prior art will be described. In the figure, reference numeral 1 designates a transceiver to which the ultrasonic probes P.sub.1 and P.sub.2 are connected respectively, 2 a clock pulse generator and 3 a gate circuit which passes the clock pulse from the clock pulse generator 2 to an up-down counter 4 and is supplied with the gate signal from the transceiver 1. The gate circuit 3 is opened or is made conductive for the time period starting from the transmission of an ultrasonic wave pulse from one of the ultrasonic probes P.sub.1 and P.sub.2 until the reception of the ultrasonic signal by the other of the ultrasonic probes P.sub.1 or P.sub.2, that is, the time period which is required for the ultrasonic wave to travel through a fluid to be measured between the positions where the ultrasonic probes P.sub.1 and P.sub.2 are located. The up-down counter 4 produces an output which is proportional to the difference between the propagation times of ultrasonic waves through the fluid between the probes P.sub.1 to P.sub.2 and to the direction between P.sub.2 and P.sub.1. The output from the up-down counter 4 is fed to a memory circuit 5 and stored therein. The output from the memory circuit 5 is applied through a D-A converter 6 to an indicator 7 which then indicates the flow rate. The output from the memory circuit 5 is also applied through an accumulating circuit 8 to an integrating meter 9 which indicates an integrated value of flow rate which gives quantity of flow. In FIG. 2, reference numeral 10 indicates a timer which is controlled by the output from the clock pulse generator 2 and controls changes of adding and subtracting operations of the up-down counter 4 and also controls the transfer of the output from the memory circuit 5.
The ultrasonic flow meter using the time difference method described above has the advantage that it is simple in construction as compared with that of the frequency difference method described later on, but it has the drawback that the measured value is apt to be affected by the velocity of sound through the fluid to be measured.
Next, the theory of the frequency difference method will be described. The frequency difference method usually employs a so-called sing-around method.
In FIG. 1, if an ultrasonic pulse wave is emitted from the ultrasonic probe P.sub.1, it is received by the other ultrasonic probe P.sub.2 after a predetermined time interval. Then, the received signal is converted to a corresponding electrical pulse signal which is amplified and then applied to the probe P.sub.1 and emitted as an ultrasonic pulse wave. Thus, the ultrasonic pulse wave and electric pulse signal sing around the path from the probes P.sub.1 to P.sub.2 through the fluid F, the probe P.sub.2 and an electrical circuit (not shown in FIG. 1). In this case, the time interval required for the pulses to circulate the path is substantially determined by the time period within which the ultrasonic wave propagates through the fluid. Accordingly, the repetition frequency f.sub.1 is given by the following equation (3): ##EQU3##
Similarly, a repetition frequency f.sub.2 in the case where the ultrasonic pulse and electrical pulse sing around the path from the probes P.sub.2 to P.sub.1 through the fluid F, the probe P.sub.1 and the electrical circuit can be expressed as follows: ##EQU4##
Thus, a difference .DELTA. f between the frequencies f.sub.1 and f.sub.2 can be expressed as follows: ##EQU5##
Accordingly, it will be apparent from the equation (5) that the flow velocity u of the fluid F can be obtained by measuring the difference .DELTA. f based upon the equation (5).
Since the equation (5) has no factor relating to the sound velocity c in the fluid F, the method is not influenced by the sound velocity changes in the fluid F, that is, no measurement error is caused even if the sound velocity c in the fluid F changes. For this reason, this method is primarily used in the art.
An ultrasonic flow meter of the frequency difference method will be now described with reference to FIG. 3 in which reference numerals corresponding to those in FIG. 2 indicate the corresponding elements.
In the example shown in FIG. 3, the ultrasonic pulse wave and electrical pulse are circulated from one of the ultrasonic probes P.sub.1 and P.sub.2 to the other thereof by the transceiver 1 and electrical pulses in one period or more within a predetermined time interval are fed from the transceiver 1 to a frequency multiplier 12. The frequency multiplied electrical pulses are then applied to the up-down counter 4, which produces an output corresponding to the difference between the multiplied pulse numbers in a predetermined time period in the direction from the probes P.sub.1 to P.sub.2 and vice versa. The output from the up-down counter 4 is supplied to the memory circuit 5 and then stored therein. The output from the memory circuit 5 is applied through the D-A converter 6 to the indicator 7 and indicated and also through the accumulating circuit 8 to the integrating meter 9 to be indicated as an integraded value or flow amount. In this example, the timer 10 controls the transceiver 1, the up-down counter 4 and the memory circuit 5.
The prior art ultrasonic flow meter shown in FIG. 3 has the advantage that it is not aflected by the sound velocity in the fluid which may change in accordance with the changes of the temperature, composition and the like of the fluid through which the sound travels, but has the drawback that its circuit construction becomes complicated as compared with that of the flow meter belonging to the time difference method and hence it becomes expensive.
Further, since the meter shown in FIG. 3 is required to increase the multiplication (for example 300.about.2000 times) so as to obtain a predetermined resolution, the meter, especially its circuit becomes unstable. Also, great skill is required to set the scale factor of the circuit, much time is needed for adjustment of equipment and changing the scale factor, the measuring time becomes long (for example, 1.about. several seconds), and the response deteriorates.